Energy efficiency with AFO use varies in cerebral palsy patients

Longer strides lead to increased work.

Ankle foot orthoses (AFOs) are regularly prescribed to children with walking disabilities resulting from cerebral palsy, yet the effects of AFOs on energy recovery and work during gait are still unclear.

In a study published in the March 2012 issue of Clinical Biomechanics, researchers from the University of Virginia in Charlottesville found that although AFOs can reduce the work needed to walk for some children with cerebral palsy, they do not do so for others.

Most children had an increase in stride length, gait speed, and energy recovery, but the increase in energy recovery did not decrease the external work. Additionally, there was a great variability in the measures related to work—while use of the AFO reduced work for some children, others experienced no change or a negative effect.

“The main culprit is that longer steps result in greater energy excursions, which the improved gait mechanics could not overcome,” said Bradford Bennett PhD, research director of the Motion Analysis and Motor Performance Laboratory and assistant professor of research in the Department of Orthopaedic Surgery at the University of Virginia School of Medicine in Charlottesville.

To examine the effects, researchers observed 21 children with spastic diplegic cerebral palsy who wore AFOs. A full body marker set of 38 markers was attached to each patient, who then walked with and without their prescribed AFOs at a self-selected speed while three-dimensional data were collected.

Devices worn by the study participants included solid AFOs and hinged AFOs with a plantar flexion stop. The results for the two groups were combined, as there was no statistically significant difference between them, Bennett said.

AFO use was associated with significant increases in stride length and gait speed, but the effect on work varied within the group. Eight of the 21 patients experienced a decrease in work of more than 10%, with an average reduction of 31%. But six patients experienced an increase in work of more than 10%; their average increase was 24%.

Overall, although AFO-assisted gait was found to be more pendular with an improved recovery factor, the external work on the user’s center of mass was not reduced as the improved mechanics were offset by increased variations of the potential and kinetic energies.

Maximum values for potential and kinetic energies were higher and minimum values were lower throughout the gait cycle for the AFO condition than the barefoot condition. This was not surprising, Bennett said, as increased stride length is known to increase energy variation. The large variations in the measured energies were offset by improved energy recovery, yet the improvements were not enough to decrease the net work.

Although these findings are not necessarily new, such research may help physicians in prescribing AFOs in the future.

“It’s worthwhile to do work like this because the more we understand how braces work, the better we are at prescribing patient by patient,” said Jon Davids MD, assistant chief of orthopedic surgery at Shriners Hospitals for Children—Northern California in Sacramento. “Even though they’ve been used for many years, there are still many things we don’t understand about what they do and don’t do. This kind of research helps us understand these things better.”

Energy efficiency should be considered in the context of a patient’s gait mechanics, Davids said.

“It’s hard to always say which quality is the clinical priority,” he said. “If an AFO is extremely energy efficient, that would be the deciding factor. However, if there is only a subtle difference in energy expenditure but it improves stability or other aspects of gait, gait mechanics would be the deciding factor.”

According to Bennett, energy expenditure is still important even if gait mechanics are improving.

“For most of these children, and as they get older, it is the high cost of walking that is the limiting factor,” he said. “Doubling or tripling the cost of locomotion is similar to the cost if one ran very hard from place to place.”

In the end, however, it’s up to the patient.

“A brace must provide a functional benefit for children to embrace it and wear it,” Davids said. “If it doesn’t improve whatever their clinical problem is but maybe is energy efficient, it will be rejected. Comfort and function are what determine a successful outcome.”

Samantha Rosenblum is a journalism student at Northwestern University in Evan­ston, IL.